1. Field of the Invention
The present invention relates generally to communications networking, and more specifically, to modulating bandwidth in a communications network.
2. Background Art
As common in most communications networks, cable television communications networks constantly must be reconfigured to provide adequate bandwidth to a host of users and services. In a conventional cable television communications network, a communications device (such as a modem) requests bandwidth from a headend device prior to transmitting data to its destination. The headend device allocates bandwidth to the communications device based on availability and the competing demands from other communications devices. Typically, bandwidth is available to transmit signals downstream to the communications device. However in the upstream, bandwidth is more limited and must be arbitrated among the competing communications devices. With the advent of personal television services and other non-traditional services (such as, telephony and Internet communications), upstream bandwidth is becoming even more limited.
The downstream channel carries the information used by the communications devices to manage upstream modulation and arbitrate bandwidth requirements. To govern upstream transmissions, the headend device prepares an upstream channel descriptor (UCD) to configure the properties or operating characteristics of the upstream. The UCD provides instructions that, in essence, partition the upstream into multiple regions. In asynchronous networks, the UCD defines one or more upstream channels, and separates each channel into distinct time slots. The communications devices utilize the time slots to transmit various types of upstream bursts.
In addition to specifying a slot within an upstream channel that is used to carry an upstream burst, the UCD also stipulates the slot structure. The slot structure represents the granularity of bandwidth allocation. The slot structure can be defined as a minislot as specified by the Data Over Cable System Interface Specification (DOCSIS) for governing cable communications. A minislot count is used to denote each individual time slot.
After the upstream properties have been configured by an UCD, the headend device generates MAP messages to instruct the communications devices on utilizing the assigned regions. These instructions are dictated in information elements (IE) delineated in a MAP message. In a DOCSIS-compliant system, MAP messages are sent downstream to provide information about time slot assignments for the upstream channels associated with a downstream channel. In other words, the MAP messages assign one or more upstream channels to a specific communications device. The MAP messages also specify a time that may be used by the communications devices to transmit on an upstream channel and the type of data that may be transmitted. Moreover, MAP messages are used by the headend device to predict the arrival of data from a communications device, the source of the data, and the type of data expected.
From time to time, the headend device must modulate the upstream characteristics and/or parameters to increase throughput and/or mitigate noise and corruption in the transmissions from one or more communications devices. One technique used by the headend device is changing or reassigning the upstream channels. For example, if a communications device receives poor service from one upstream channel, the headend device instructs the communications device to use a second upstream channel having different operating characteristics and/or parameters. However, this technique may not be effective if the second upstream channel operates at an equal or worse quality of service. Additionally, switching the communications device to a second upstream channel may adversely impact the services to the communications devices currently operating over the second upstream channel.
A second solution would be to change the operating characteristics and parameters of the existing upstream channel. More specifically, the headend device could generate a new UCD to alter the slot structure. Since the slot structure is being altered, the communications devices must also be commanded to restart the minislot count used to denote each time slots. In other words, once the revised slot structure is implemented, the communications device must restart counting and/or identify the revised minislots with the revised minislot count.
Additionally, the headend device, itself, must also be prepared to implement the revised slot structure and minislot count. The headend device must be able to anticipate the arrival of upstream bursts from the communications devices. Thus, the headend device needs to know the expected size and time-of-arrival of the incoming bursts. MAP messages are used by the headend device to anticipate the arrival of bursts of a specified size. The headend device also generates a minislot count that is paired with the MAP messages. Thus, the headend device uses the minislot count to anticipate the time-of-arrival of the incoming burst.
However, current DOCSIS-compliant systems do not provide an efficient protocol for generating and implementing a new minislot count for a revise slot structure. For example, the headend device typically includes an upstream demodulator interface that receives upstream bursts from the communications devices. The upstream demodulator interface uses the minislot count and MAP messages, produced at the headend device, to plan for the burst's arrival. The upstream demodulator interface also expects each new minislot count to be a sequential increment from the previous minislot count. For instance, if the previous minislot count is “63,” the upstream demodulator interface expects the next minislot count to be “64.” Moreover, if the upstream demodulator interface receives a new minislot count that is not sequentially related to the previous minislot count, the new minislot count can be rejected.
However, if the headend device changes the slot structure, the headend device also has to re-initiate the minislot count. Since the revised slot structure is either smaller or larger than the current slot structure, the revised minislot count is generated at either a faster or slower rate than the original minislot count. This leads to the distinct possibility that at any given point in time, the actual value of the revised minislot count will differ from the original minislot count. Hence, at the switchover point to the revised minislot count, the actual value of the revised and original minislot counts will differ.
This is problematic because the upstream demodulator interface will receive a different minislot count than expected. Referring to the previous example, at the switchover point, the original minislot count may be at “64,” but the revised minislot count may be at, for example, “32” or “128.” If the upstream demodulator interface receives a minislot count of “32” after receiving the value “63,” the upstream demodulator interface considers the new minislot count to be an error because the value is too old. The upstream demodulator interface may consider the new minislot count to be a delayed value that is no longer valid. Similarly, if the upstream demodulator interface receives a minislot count of “128” after receiving the value “63,” the upstream demodulator interface can consider this value to be faulty because it is too far in advance.
As can be seen, if the upstream demodulator interface discards perfectly valid minislot counts or expends a considerable amount of time validating the data, the resulting effect is a degradation in network efficiency and performance. Upstream bursts can be loss or delayed which is intolerable for high quality services such as telephony.
Therefore, a protocol for modulating upstream properties is needed to address the above problems.